1. Field of the Invention
The present invention relates to a method in connection with vaporizing an anaesthetic, in which an anaesthetic vaporized in a vaporizing chamber is mixed with fresh gas and delivered to be inhaled by a patient. The invention also relates to an arrangement in connection with vaporizing an anaesthetic.
2. Description of the Related Art
The invention thus relates to anaesthesia and particularly to vaporizing a liquid anaesthetic. The vaporized anaesthetic is mixed with fresh gas, which may comprise oxygen and nitrous oxide. Fresh gas is obtained from the hospital gas supply system by forming a mixture of various gases, for instance. The mixed flow comprised of fresh gas and vaporized anaesthetic is administered for the patient to inhale when anaesthetizing the patient, for instance.
Vaporization of a liquid anaesthetic consumes thermal energy in a similar manner as vaporization of other liquid substances does. In conventional vaporizers previously known in the field, sufficient heating energy is transmitted to the liquid by convection from the constructions of the vaporizer and from ambient air. The temperature of the vaporizer, and of the anaesthetic contained therein, tends to decrease in vaporization. When the temperature of the anaesthetizing liquid drops, also the pressure of vapour evaporated therefrom, and simultaneously its concentration within the vaporizer, decreases. This would cause the concentration of the administered anaesthetic to decrease, if no compensation according to temperature were carried out.
A new anaesthetic, desflurane, has recently been developed in the field. The physical and physiological properties of desflurane are markedly different from those of conventional anaesthetics. Desflurane has a boiling point of 23.5.degree. C. at normal air pressure, while that of other anaesthetics is about 50.degree. C. Furthermore, desflurane has a much stronger dependency of vapour pressure on temperature at normal operating temperatures than the anaesthetics previously employed. For example at 20.degree. C., desflurane has a vapour pressure of 32 mmHg/.degree.C. whilst that of halothane, one of the previously used anaesthetics, is 10 mmHg/.degree.C. Moreover, the ranges of use that are possible for desflurane are broader, 0-18%, than those for previously employed anaesthetics, which are 0-5%. The above factors require new methods for safe and reliable vaporization of desflurane.
In the conventional vaporizer of the by-pass type, part of the fresh gas is passed via a conduit to a vaporizing container containing an anaesthetic liquid, and part of it bypasses the container. The vapour saturated with anaesthetic issuing from the container is recombined with the by-pass flow, and the mixture is then delivered to the patient. The concentration of anaesthetic may be varied by adjusting the distribution ratio with a valve, for example. Also temperature compensation is based on varying the distribution ratio as a function of temperature. When the temperature and concurrently the vapour pressure of the anaesthetic decreases, a larger portion of the fresh gas flow is passed through the container containing anaesthetic, and when the temperature rises, the flow through the container is diminished.
If desflurane is vaporized with the conventional by-pass: method, temperature changes are more difficult to compensate for than with previously employed anaesthetics. This is due to the following factors:
Desflurane exhibits a high dependency of vapour pressure on temperature, as previously stated. If the liquid temperature is below the boiling point, the temperature of the anaesthetic liquid decreases when the anaesthetic is vaporized, and simultaneously the anaesthetic concentration of the vapour contained within the vaporizing container decreases. This decrease in concentration as a function of temperature is about three times as high with desflurane as with other anaesthetics.
Desflurane concentrations in fresh gas to be administered to a patient are about four times those of conventional anaesthetics. In consequence, the desflurane vaporizer must be capable of producing four times the quantity of vapour needed with other vaporizers. Since the vaporization heat of desflurane is of the same order as that of other anaesthetic agents, also the heat energy required for vaporization is about four times that required for the conventional anaesthetic agents. If no exterior energy were supplied to the vaporizing container, also the temperature drop produced by vaporization would be about four times the temperature drop encountered with conventional agents.
From the above it can be seen that the temperature of desflurane drops more rapidly than that of other anaesthetics, and that the temperature drop has a more drastic effect on the concentration than in vaporizers for other anaesthetics.
In principle, three solutions have been developed for vaporizing desflurane. In the first, the liquid desflurane is electrically heated to a certain temperature above the boiling point, and the temperature is maintained constant by active heating. In that case, the vapour to be administered consists entirely of desflurane. Since the pressure in the vaporizing chamber increases with the increase of temperature, anaesthetic can be supplied directly from the container without any fresh gas flow through the container. Such an arrangement is disclosed in European published application 0 454 390 and British published application 2 253 149, for instance.
In the above method, no compensation of temperature changes is needed, as the temperature is maintained constant and the vapour to be administered consists entirely of anaesthetic. The greatest drawback of the method is the high consumption of electrical energy. As the liquid has a temperature above ambient, all energy must be supplied to the vaporizer by electrical means to produce a sufficient amount of vapour and also to maintain the temperature in the vaporizer constant, since the entire vaporizer is cooled to ambient air. Furthermore, on account of the heating the vaporization cannot be commenced until a sufficient vaporizer temperature has been achieved.
In another known arrangement, an anaesthetic liquid is cooled below the boiling point, and the temperature of the liquid is maintained constant during vaporization by active cooling and heating. In this arrangement, the conventional by-pass method must be used to administer the anaesthetic. Such an arrangement is disclosed in U.S. Pat. No. 5 168 866, for example.
In the above prior art method, the temperature effects of desflurane are managed by actively maintaining the temperature constant. The problems of the method are essentially the same as in the above method based on heating. Cooling consumes even more energy than heating, since the efficiency of cooling elements is poor. It is further to be noted that the vaporizer cannot be operated during cooling.
A third prior art arrangement is disclosed in European published application 0 545 567, for example. This arrangement is a conventional by-pass vaporizer. The method does not stabilize the liquid temperature at all; by determining the gas flow issuing from the vaporizing chamber and also its anaesthetic concentration in some way, the anaesthetic can be dosed by regulating the flow ratio. The temperature of the anaesthetic is not controlled in this method.
The above method manages the temperature effects of desflurane by measuring the physical status of the system, and the flow through the vaporizing chamber is regulated on the basis of the measuring results. The problem with this known method consists in the high requirements set on the temperature measuring accuracy and the wide control range required for the flow ratio, resulting from the above-stated temperature effects of desflurane.